1. Field of the Invention
The present invention relates to conductive carbon particles having improved performance as a filler material and improved dispersability within a matrix material. More particularly, the present invention involves compositions of carbon particles having coatings of conductive polymer which electrically and physically interact with the carbon particle to protect the carbon particle from conductive failure and provide an effective interface between the carbon particle and a matrix material, such as a polymer.
2. Description of Related Art
Carbon has found extensive utility recently in a variety of applications where its electrical conductivity, low density, low cost, and ease of processing are especially attractive. In particular, carbon in the form of particulates such as carbon black is widely used as a conductive filler material for polymers utilized to fabricate housings for electronic equipment, floor mats for electronic assembly areas, volatile chemical and fuel transport lines, conductive adhesives, electronic components and storage bins. The usefulness of carbon black in these applications is primarily attributed to its low density and its ability to dissipate accumulated static charges as well as prevent the build-up of static charges.
The static dissipation property of carbon prevents possible catastrophic explosions and/or fires which can result when charges accumulate and discharge in the form of sparks during the movement of fuels or volatile chemicals through polymeric transport lines. In the case of its utility in electronic housings, the static dissipation property prevents potentially damaging loss of data or equipment malfunction caused by discharging sparks from static charges which have built-up on the housing. Additionally, carbon filled composites can be pressed and molded to form and retain almost any shape.
Additionally carbon particulates or monolithic devices fabricated from carbon are useful in fabricating batteries and electrodes. Again, the low density and electrical conductivity of carbon makes this material attractive in applications where light weight conductive materials are preferred.
One problem associated with relying upon the conductive properties of carbon over an extended period of time relates to its highly adsorbent surface. In addition to being useful for its conductive properties, carbon is suitable for purifying and removing unwanted and especially highly colored components in liquid mixtures. Because the surface of carbon particles is a particularly good adsorbent for organic compounds, it is routinely utilized to remove soluble and insoluble organic impurities in aqueous systems. Unfortunately, this superior ability to adsorb compounds within its environment frequently results in a loss or significant reduction in the conductivity of carbon exposed to certain environments.
For example, chemical and fuel transport lines fabricated of polymers filled with carbon particles can become non-conductive as a result of the adsorption and interaction of the carbon particles with the chemical or fuel. Similarly, conductive carbon particle filled polymeric devices utilized in reactive gas or other hostile environments can lose their conductivity once exposed to the reactive gas or hostile environment for a sufficient length of time.
Conductive organic polymers have also found utility in the fabrication of devices having static-dissipation properties and in the fabrication of electrodes. Since these polymers are considerably more costly than carbon, are not structural materials and do not have the physical characteristics to be effective filler material, conductive polymers are limited in their practical static-dissipation usefulness.
In the case of their usefulness in electrochemical cells, however, conductive organic polymers have gained widespread attention. For example, conductive forms of polyaniline are widely recognized for their utility in electrochemical cells as a cathode or an anode. Additionally, in some electrochemical applications, conductive polymers are combined with carbon, to provide carbon filled polymeric composites for fabricating electrodes. The conductive polymer is the portion of the electrode which charges and discharges and is the primary functioning component of these conductive polymer/carbon electrodes. For this reason, the combination of carbon and polymer contains large amounts of polymer and relatively small amounts of carbon, the carbon being present primarily as a conductive filler material which provides strength and a substance to the mixture while maintaining the conductive nature of the polymer. Secondarily, the carbon may serve as a pathway of current flow as the conductive polymer is oxidized/reduced.
Typically, battery components of carbon and conductive polymer composites are prepared by merely mixing the conductive polymer and carbon using relatively large amounts of polymer and smaller amounts of carbon and then pelletizing the mixture into a homogeneous monolithic composite of carbon in a polymeric matrix. For example, U.S. Pat. No. 4,803,138 discloses polyaniline electrodes of pressed polyaniline powder, polytetrafluoroethylene and about 10 wt % carbon black. The amount of electrical interaction between carbon filler and conductive polymer in these composites is limited by the degree of physical intimacy between the filler and polymer obtained during the pressing operation.
Another similar application involves preparing uniform dispersions of carbon in pressed carbon electrodes. This application, disclosed in Japanese Patent Bulletin (A) 1987-64828, involves synthesizing non-conductive polyaniline in the presence of carbon to provide more uniform carbon compositions and improved mechanical strength for carbon electrodes prepared from the compositions.
Accordingly, it is an object of the present invention to provide electrically conductive compositions having improved resistance to loss of conductivity.
It is further an object of the present invention to provide electrically conductive compositions of carbon particles which maintain their physical and electrical characteristics in the presence of chemically reactive and hostile environments.
It is additionally an object of the present invention to provide electrically conductive compositions of carbon particles for use as filler material in formulations having anti-static properties.